12/03/2007

PARIS (AFP) — The greenhouse effect is causing Earth's zone of tropical climate to creep towards the poles, according to a study whose release on Sunday coincided with the eve of a major UN conference on climate change.

The poleward expansion of the tropics will have far-reaching impacts, notably in intensifying water scarcity in the Mediterranean and the US "Sun belt" as well as southern Africa and southern Australia, it warns.

The paper, appearing in a new journal, Nature Geoscience, is an overview of the latest published research into atmospheric systems at the tropics.

For cartographers, the tropical belt is defined quite simply by the tropics of Cancer and Capricorn, at latitudes 23.5 degrees north and south of the equator.

For climatologists, though, where the tropics end is fuzzier because of the complexity of a powerful high-altitude wind pattern, known as the Hadley circulation.

These powerful jet streams are what determine precipitation patterns of the tropics, which is characterised by lots of rain in the central part of the belt near the Equator and by dryness at its fringes.Some years ago, the first credible computer simulations predicted that, as the Earth warmed, the Hadley jet streams and their associated wind and rainfall patterns would move poleward.

Under the most extreme scenario, the tropics were on average predicted to expand by about two degrees latitude, equivalent to around 200 kilometres (120 miles) over the 21st century.

The new paper looks at a batch of recent studies based on five different types of measurement from 1979-2005.

It concludes that this change in the tropical jet streams has already happened -- and the worst-case scenario has already been surpassed.

"Remarkably, the tropics appear to have already expanded -- during only the last few decades of the 20th century -- by at least the same margin as models predict for this century," it says.

"The observed widening appears to have occurred faster than climate models predict in their projections of anthropogenic [man-made] climate change."